Reduction mechanisms of ash deposition in coal and/or biomass combustion boilers

Some ash particles in solid fuels adhere on heat exchanger tube surfaces inside coal and/or biomass combustion boilers. The authors have already proposed a surface treatment on tubes, using a thermal spraying technique, to reduce ash deposition. Understanding reduction mechanisms of the ash deposition is necessary to evaluate effects of the surface treatment technique on the reduction of ash deposition. The reduction mechanisms of the ash deposition were elucidated due to physical and chemical aspects, measuring adhesion forces between the ash particles and some alloy specimens of the tube at high temperature under the simulated boiler conditions. As a result, the adhesion force increased with time and depended on both the ash types and the alloy specimens. The thermal spraying of Ni-alloy, in particular, could reduce the adhesion force. Moreover interface reactions between the ash particles and the alloy specimen played an effective role in increasing the adhesion force, alkali metal compounds in the ash samples also related to an increase of the adhesion force. Fe, which was one of the main alloy elements, diffused into the ash deposition layer beyond the interface. This observation result suggested that the interface reactions of the ash particles with the alloy caused an increase of the adhesion force.